Experience gained through crash testing a variety of vehicles has revealed that, in many vehicles and in many types of crash situations, the crash sensor detects the crash too late for optimum deployment of an inflatable occupant restraint system. Delayed sensing of the crash event may occur because the sensor is mounted too far away from the crush zone, or the crash pulse is attenuated or delayed because of the collapse of soft, front-end vehicle structures. The only way to avoid such delay is to mount the sensor in the crush zone, where the information about the crash arrives earliest.
Analysis of vehicles after accidents indicates that the vehicle structure on which the sensor is mounted is often pushed back and/or rotated by the crash. Thus, conventional inertial sensors, such as ball-in-tube sensors or roller/spring sensors, when used in the crush zone, are subject to being destroyed or disconnected by the crash before they can signal the event
One proposal to avoid the undesirable aspects of crush zone mounting is to replace inertial sensors with rugged switches which are closed by the impinging structure of the vehicle as it is pushed back by the force of the crash. Although this expedient works in some crash situations, it is limited by the fact that if the switch is not in the crush zone, the switch cannot detect the crash. For example, if the switch is mounted on body panels, it cannot detect, for example, the undercarriage of a vehicle hanging up on a low post or falling into a hole, in which situation the occupant would not be protected.